The demand for water for hydraulic fracture stimulation (hydraulic fracturing or hydrofracing) of natural gas reservoirs is ever increasing. For example, unconventional gas such as shale gas requires multiple large slickwater fracturing stimulations applied to horizontal wells in order to enable the recovery of this resource. To meet these demands, it would be desirable to be able to use otherwise unusable water such as certain well water or discharge water from industrial processes without a significant environmental, economic, social, heritage or health effect.
It is important that water used for hydraulic fracture stimulation (“frac water”) is compatible with the water present in the shale formation. Often, shale water is very high in calcium (upwards of 14,000 mg/L or more), magnesium, barium and strontium. Thus, it is important that frac water has a reduced amount of bicarbonate to prevent scaling. Scaling is primarily caused by the formation of calcium carbonate, magnesium carbonate, barium carbonate, and the like, which salts can deposit on surfaces such as the inside of pipes and build up until the passageway of the pipe is severely restricted.
However, some well water in certain areas contains significant amounts of hydrogen sulfide (e.g., some as high as 60 mg/L or parts per million [ppm] or higher), which may lead to health risks (death), corrosion problems, odor problems and the like. Further, in many cases the well water quality is saline, having total dissolved solids [TDS] concentrations ranging from about 4,000 mg/L to 25,000 mg/L TDS and in some cases 40,000 mg/L or higher. It is not uncommon for certain well water to have very high amounts of bicarbonate, in some instances, upwards to 3,300 mg/L or more. Thus, these water sources would be deemed unusable for domestic or agricultural purposes but could be a good water source for gas field development.
Many innovations in water treatment are required to enable saline sour water to be suitable for use in shale gas completions. Ideally, to ensure a safe work environment and protection of fracturing equipment, the acceptable operational hydrogen sulfide (H2S) level should be close to zero ppm, for example, the maximum allowable H2S concentration preferably being less than 45 ppb in the liquid phase. Further, the amount of bicarbonate should be reduced to prevent scaling problems.
Thus, there is a need for an efficient process and process line for removing hydrogen sulfide and bicarbonate from water to render the water suitable for hydraulic fracture stimulation. More particularly, a process and process line is needed that may accomplish one or more of the following: removal of H2S, preferably, irreversible removal of H2S, prevention of scaling, no precipitates or liquid waste streams, closed loop system (i.e., not open to atmosphere) and minimum release of H2S, in order to produce water that is suitable for use in hydraulic fracing.